Development of non-lethal methods for determining age and habitat use of sawfishes from northern Australia - Final Report
Field IC, Meekan MG and Bradshaw CJA
Prepared for the Department of the Environment, Water, Heritage and the Arts, April 2009
About the report
Measuring the life history traits of fish provides important information to estimate rates of population change that are essential for species conservation and fisheries management. Such traits ultimately determine a populations rate of recovery, sustainable harvest and risk of extinction. Of the various traits, age is a key variable that must be estimated to calculate parameters such as population growth rates, mortality, and productivity. Furthermore, life history information should be combined with habitat use patterns and movement data because these determine the degree to which sub-populations are connected. Low connectivity (genetic exchange among sub-populations) increases the risk of extinction. In the past decade it has become clear that marine biodiversity worldwide is under ever-increasing threat, primarily as a result of over-harvesting, pollution and the direct and indirect impacts of climate change. One taxonomic group of marine fishes that has come under increasing scrutiny in terms of extinction risk from these processes is chondrichthyans (sharks and rays and chimaeras), especially those species that live in coastal environments and have critical habitat requirements, such as sawfishes.
There are four species of sawfish in Australia: freshwater (Pristis microdon), green (P. zijsron), dwarf (P. clavata), and narrow (Anoxypristis cuspidata). P. microdon and P. zijsron are listed as Vulnerable under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Although globally endangered, Australian populations may be relatively abundant and the least threatened compared to populations elsewhere and therefore, they provide a unique opportunity to learn and understand the dynamics of arguably the most intact population anywhere in the world. We aimed to develop a non-lethal aging technique for sawfishes and determine if stable isotope analyses could be used as a tool for identifying population sources and movement patterns.
To date, most methods to estimate age have required lethal sampling. We provide evidence that the bands on the rostral teeth of narrow sawfish can be used to estimate age in a non-destructive manner. In this preliminary study, we compared growth rings from vertebrae and rostral teeth for 16 Anoxypristis cuspidata. Teeth were stained with silver nitrate and increments counted. There was a positive relationship between size and estimated age of individuals. However, the estimated age of the individual determined from the rostral teeth became more uncertain with increasing age and size. Relative estimates of age from teeth were similar to ages determined from counts of vertebral growth bands, but there was some evidence to suggest that ages were underestimated for older sawfishes relative to those obtained from vertebrae. More research is needed to reduce variation in counts and to verify if this technique can be applied to other species.
We also examined the stable isotope composition of sawfish rostral teeth to determine whether chemical fingerprints could be used as tags to identify populations. We used a solution-based inductively coupled plasma-mass spectrometer (ICP-MS) to obtain chemical signatures. We found regional differences in the stable isotope composition of Anoxypristis cuspidata for individuals from the Gulf of Joseph Bonaparte in the Northern Territory and from the Gulf of Carpentaria in Queensland. We found some evidence to suggest that local differences in stable isotope composition exist for Pristis microdon and P. clavata caught along the Queensland coast, but results were uncertain due to small sample sizes. When individuals of all species were combined (assuming that stable isotope physiological integration processes were the same), we found a difference in chemical signatures that might indicate that freshwater sawfish (Pristis microdon) and dwarf sawfish (P. clavata) have overlapping habitats or diet and that these differ to those of Anoxypristis cuspidata. Further investigation of fine-scale spatial differences is required to refine and verify our findings along with new studies to understand the processes that influence the integration of stable isotopes into body tissues.